Self-similar vortex-induced vibrations of a hanging string

International audienceAn experimental analysis of the vortex-induced vibrations of a hanging string with variable tension along its length is presented in this paper. It is shown that standing waves develop along the hanging string. First, the evolution of the Strouhal number S t with the Reynolds number R e follows a trend similar to what is observed for a circular cylinder in a flow for relatively low Reynolds numbers (32 < Re < 700). Second, the extracted mode shapes are self-similar: a rescaling of the spanwise coordinate by a self-similarity coefficient allows all of them to collapse onto a unique function. The self-similar behaviour of the spatial distribution of the vibrations along the hanging string is then explained theoretically by performing a linear stability analysis of an adapted wake-oscillator model. This linear stability analysis finally provides an accurate description of the mode shapes and of the evolution of the self-similarity coefficient with the flow speed

Flutter of long flexible cylinders in axial flow

International audienceWe consider the stability of a thin flexible cylinder considered as a beam, when subjected to axial flow and fixed at the upstream end only. A linear stability analysis of transverse motion aims at determining the risk of flutter as a function of the governing control parameters such as the flow velocity or the length of the cylinder. Stability is analysed applying a finite-difference scheme in space to the equation of motion expressed in the frequency domain. It is found that, contrary to previous predictions based on simplified theories, flutter may exist for very long cylinders, provided that the free downstream end of the cylinder is well-streamlined. More generally, a limit regime is found where the length of the cylinder does not affect the characteristics of the instability, and the deformation is confined to a finite region close to the downstream end. These results are found complementary to solutions derived for shorter cylinders and are confirmed by linear and nonlinear computations using a Galerkin method. A link is established to similar results on long hanging cantilevered systems with internal or external flow. The limit case of vanishing bending stiffness, where the cylinder is modelled as a string, is analysed and related to previous results. Comparison is also made to existing experimental data, and a simple model for the behaviour of long cylinders is proposed

Coupled Inline-Cross Flow VIV Hydrodynamic Coefficients Database

Vortex Induced Vibrations (VIV) cause major fatigue damage to long slender bodies and have been extensively studied in the past decades. While most of the past research focused on the cross flow direction, it was recently shown that the inline motion in the direction of the flow has a major impact on the fatigue life damage due to its higher frequency (second harmonic) and more importantly, its coupling with the crossflow motion, which triggers a third harmonic stress component in the cross flow direction. In this paper, the coupled inline-crossflow VIV problem is addressed from semiempirical modeling of fluid forces. Extensive fine grid forced inline-crossflow VIV experiments were designed and carried out in the MIT towing tank. An inline-crossflow VIV hydrodynamics coefficients database was newly constructed using the experimental results and it is expected to be useful for other semi empirical programs predicting coupled inlinecrossflow VIV in the field. Several key hydrodynamic coefficients in the database, including lift force coefficients, drag force coefficients and added mass coefficients, were systematically analyzed. The coefficients in the crossflow and the inline directions were found to have strong dependency on the phase between the inline and crossflow motions.BP-MIT Major Program

Inline-Crossflow Coupled Vortex Induced Vibrations of Long Flexible Cylinders

The inline motion of long flexible cylinders caused by Vortex Induced Vibrations (VIV) has been long neglected due to its small amplitude compared to the cross-flow response amplitude. However, the inline motion has a major impact on fatigue life due to its higher frequency (second harmonic) and more importantly, because it triggers a third harmonic stress component in the crossflow direction along with a broad-band frequency stress component. We introduce an inline response prediction module to VIVA, a VIV response prediction program widely used in the offshore industry, to be able to consequently predict the higher harmonic and chaotic VIV response characteristics of flexible cylinders. Extensive forced inline and combined inline-crossflow experiments were employed to provide hydrodynamic coefficient databases for input to VIVA, in addition to existing crossflow hydrodynamic coefficients. The Norwegian Deepwater Programme (NDP) experimental data were used to validate this prediction methodology.BP-MIT Major Projects Progra

On the efficiency of energy harvesting using vortex-induced vibrations of cables

Many technologies based on fluid–structure interaction mechanisms are being developed to harvest energy from geophysical flows. The velocity of such flows is low, and so is their energy density. Large systems are therefore required to extract a significant amount of energy. The question of the efficiency of energy harvesting using vortex-induced vibrations (VIV) of cables is addressed in this paper, through two reference configurations: (i) a long tensioned cable with periodically-distributed harvesters and (ii) a hanging cable with a single harvester at its upper extremity. After validation against either direct numerical simulations or experiments, an appropriate reduced-order wake-oscillator model is used to perform parametric studies of the impact of the harvesting parameters on the efficiency. For both configurations, an optimal set of parameters is identified and it is shown that the maximum efficiency is close to the value reached with an elastically mounted rigid cylinder. The variability of the efficiency is studied in light of the fundamental properties of each configuration, i.e. body flexibility and gravity-induced spatial variation of the tension. In the periodically-distributed harvester configuration, it is found that the standing-wave nature of the vibration and structural mode selection plays a central role in energy extraction. In contrast, the efficiency of the hanging cable is essentially driven by the occurrence of traveling wave vibrations

The Mechanics of Fast-Start Performance of Pike Studied Using a Mechanical Fish

A northern pike (Esox lucius) is capable of achieving a maximum instantaneous acceleration of 25g, far greater than that achieved by any manmade vehicle. In order to understand the physical mechanisms behind achieving such high accelerations, we have built a mechanical fish to emulate the motion of a pike, a fast-start specialist. A live pike bends its body into either a C-shaped or an S-shaped curve and then uncoils it very quickly to send a traveling wave along its body in order to achieve high acceleration. We have designed a mechanical fish whose motion is accurately controlled by servo motors, to emulate the fast-start by bending its body to a curve from its original straight position, and then back to its straight position. Furthermore, this mechanical fish is designed to be adjustable in swimming pattern, tail shape, tail rigidity, and body rigidity, making it possible to study the influence of all of these parameters on the fast-start performance. Peak accelerations of 2.0 m/s2 and peak velocities of 0.09 m/s are measured. Although the maximum accelerations and velocities observed in our mechanical fish are smaller than those of live fish, the form of the measured acceleration signal as function of time is quite similar to that of a live fish. The hydrodynamic efficiencies are observed to be around 12%, and it is shown that the majority of the thrust is produced at the rear part of the mechanical fish - similarly to the live fish. Copyright © 2011 by ASME

Re-Evaluation of VIV Riser Fatigue Damage

The paper describes a new characterization of the properties of the vortex-induced vibrations (VIV) of marine risers, which emerges from processing field and experimental data. We show that two currently employed assumptions: (a) that VIV is a statistically steady-state response containing one or several frequencies, and (b) that VIV consists of alternating dominant modes (mode-sharing), are inadequate. Instead, we find that the response either contains strong traveling wave components accompanied by high force harmonics; or consists of a chaotic wandering among several traveling and standing waves, associated with a wide-band spectrum; both types of response require careful consideration for correct fatigue evaluation. Topics: Fatigue damage, Pipeline risers, Vortex-induced vibrationBP-MIT Major Projects Progra

Nonlinear dynamics of a slender flexible cylinder subjected to axial flow

This thesis deals with the nonlinear dynamics of a vertical slender flexible cylinder supported at both ends and subjected to axial flow. The goal is to study the dynamical behaviour of this system from a nonlinear point of view, both theoretically and experimentally.A weakly nonlinear model is derived assuming that the cylinder centreline is extensible. Nonlinear Euler-Bernoulli beam theory is used for the structure and, the fluid forces acting on the cylinder are assumed to be inviscid, frictional and hydrostatic ones. The derivation of the equations of motion is carried out in a Lagrangian framework, and the resultant equations are correct to third order of magnitude. These nonlinear partial differential equations are then recast in nondimensional form and discretized by using Galerkin's technique, giving a set of nonlinear second-order ordinary differential equations.Houbolt's finite difference method and AUTO are used as two numerical methods to solve the resulting set of ordinary differential equations. The centre manifold reduction method is also used as an analytical method to study the behaviour of the system in the vicinity of the pitchfork bifurcation point.The results for a cylinder with various boundary conditions are presented in the form of bifurcation diagrams with flow velocity as the independent variable, supported by time histories, phase-plane plots, PSD plots and Poincare maps. The influence of different parameters on the behaviour of the system is also investigated.Three series of experiments were conducted on vertical clamped-clamped cylinders. In the first series of experiments, the downstream end of the clamped-clamped cylinder was free to slide axially, while in the second series of experiments, the downstream end was fixed. The influence of externally applied axial compression has also been studied in the second series of experiments. In the third series of experiments, a more flexible cylinder was used, and the effect of externally applied axial compression on the dynamic instability of the cylinder was also studied